The invention relates to a device and method for seismic drill hole measuring, and allows to perform seismic exploration of surrounding formations during oil well drilling.
Seismic measurements during oil drilling are well known: a seismic wave source is placed on the ground at a certain distance of the drill hole and produces shock waves propagating into the formation. Propagation can be either direct or indirect (through reflection on geological underground interfaces). These waves are detected by a sensor, like a hydrophone, geophone, or accelerometer. The direct and indirect propagation times allow the vertical seismic profile and the position of seismic reflectors located under the drill bit to be calculated. In order to perform these calculations of position, at least two of these sensors must be available, one of which is arranged above ground and the other one is located downhole. This example is not restrictive for seismic measurements during drilling as it is quite possible to have an arrangement the other way round according to which the seismic source is placed downhole (e.g., it may be the drill bit), and in this case, the reference sensor will be placed in the measuring tool, and the seismic sensors will be located above ground. In this configuration also, seismic wave can propagate directly towards the surface or be reflected on geological interfaces located under the drill bit and then propagate towards the surface. And in a way similar to the preceding one, it will be possible to determine the position of these seismic reflectors.
However, difficult and so far unsolved problems are posed for synchronizing, without an electric link, the measurements associated with both under and above ground sensors because of the seismic wave propagation speed and required precision. Indeed, wave propagation speed is on the order of 2000 to 5000 m per second in sedimentary ground and measurements must allow to determine the position in space of the reflectors with an inaccuracy better than 10 m from the surface. In terms of relative inaccuracy, i.e. regarding the mutual position of the various reflectors, it must be on the order of one meter. And yet, none of the conventional clocks that could be associated with the sensors and that would allow to conform to the required precision would survive to the operating conditions prevailing at the bottom of a drill hole (vibrations, shocks, temperature). However, the usefulness of such measurements is no longer to be demonstrated, indeed, they allow for instance to determine the position of cracks and reservoirs under salt domes or to clarify the interpretation of surface seismic data (in case of reservoirs the image of which is overshadowed by the presence of a gas cloud above them).
A sample prior device of this type of measurement in this technical field is described by the French patent no. 2 742 880 of Institut Franxc3xa7ais du Pxc3xa9trole, proposing alternative methods for solving the problem of measurement synchronization, as e.g. using an electromagnetic transmission or using vibratory waves propagating inside the drill string for correcting clock drifts after the event. However, although these synchronization methods allow the clock synchronization error to be maintained at several milliseconds, which is just about acceptable, it has to be added that in test synchronization is not always possible as certain formations dampen electromagnetic waves to such a degree that the signal to noise ratio at the downhole receiver does not allow the desired synchronization to be achieved. In the same way, vibratory waves propagating along the drill string can be dampened to such a degree that they can no longer be detected above ground.
U.S. Pat. No. 5,555,220 is also to be mentioned, describing a seismic probe fixed to a cable and lowered into the well. This probe requires drilling to be stopped each time it is used and it has to be brought back to the surface before drilling is resumed, which is restrictive but does not pose any serious problem of synchronizing the clock, the drift of which is low due to the tests envisaged by this specific technique being short: an ordinary quartz crystal clock enclosed in an insulating enclosure is sufficient.
The invention is a solution to this measurement synchronization problem in the specific context of long-term seismic testing, for which the events responsible for clock drift, above all overheating, are fully apparent even through an insulating enclosure, and relative drifts, which would be admissible for short durations, then produce excessive total drifts; it is based on the development of hyperstable clocks adapted to the conditions of such testing so as to have a very low output frequency drift only (between 10xe2x88x928 and 10xe2x88x929 in relative values, i.e. 1 to 10 ppb (part per billion)). Inaccuracy of synchronization is then comprised between several tenths of milliseconds and one millisecond for a test duration of several days, which is the typical duration of a test with a tool left downhole, and which allows perfectly synchronized measurement acquisitions to be performed independently of well conditions and without requiring any action slowing down the drilling process.
Although it is relatively easy to find clocks meeting this drift requirement for the surface sensor, this is quite different for the underground sensor clock, which is constantly likely to go out of order due to temperature changes, shocks, and vibrations. The development of this clock has therefore been a crucial issue of the invention.
In its most general form, the invention relates to a device allowing seismic measurements to be performed in a well during drilling, comprising, above ground, a seismic source and a recorder connected to one or several seismic reference sensors and, downhole, a seismic sensor mounted in a drill string, wherein the seismic sensors and recorders are associated with a low drift synchronized clock system, characterized in that the clock associated with the underground seismic sensor is a dual mode type clock (as described in U.S. Pat. No. 4,872,765).
Moreover, the clock associated with the tool seismic sensor is enclosed in an enclosure (of Dewar type or made of insulating materials) including a thermal regulation means so as to allow clock temperature control.
The thermal regulation system normally comprises a heating means, but it is also possible to use a cooling means (e.g. like Peltier modules) or even a combination of both means. The control temperature can be varied by the user, it is then possible to choose a temperature value slightly higher than that of the well in order to minimize power input required for thermal regulation.
The clock of the reference sensor located above ground can be a quartz crystal clock preferably maintained at a temperature determined according to a frequency stability area of the quartz.
A method in accordance with the invention thus consists in installing a seismic source, an above ground seismic reference sensor, a downhole seismic sensor inside a tool integrated in a drill string, providing low drift clock sensors, lowering the tool into the well and sending seismic waves to the sensors via the formations to be investigated; it is characterized in that the well clock is maintained at a given but variable temperature, and in that clock synchronization is performed when the given temperature is modified.
According to another specific method, a unique synchronization, prior to testing, is performed before the tool is lowered into the well, and the temperature of the well sensor clock is maintained at a uniform value.
The invention also implies using techniques for synchronizing clocks, starting measurement recordings, and retrieving the recorded data during the test campaign, when the tool is engaged in the well. Some of these techniques are known, but will be recalled briefly in this document.